It has previously been proposed to use a sample having temporary magnetic remenance to perform a thermodynamic cycle in which a varying magnetic field is used to produce a magnetic flux in the sample during a first part of the cycle, with the field being removed during a second part of the cycle, leading to collapse of the magnetic domains in the sample and the creation of an independent magnetic flux. It has been proposed to use this principle to convert heat energy into electricity, and also for refrigeration.
An example of a system of this type is described in International Publication No. WO 00/64038 entitled “Thermodynamic Cycles and Method for Generating Electricity” and filed Apr. 19, 2000. The contents of WO 00/64038 are hereby incorporated by reference. In the document, an example of the field excitation apparatus is given as being produced by a rotating permanent magnet to produce a largely sinusoidally-varying field. High-permeability materials are placed in proximity to the sample to ensure that the rise and fall of the magnetic flux in the sample are very sharp compared to the overall cycle time, so that over each pulse the flux variation in the sample resembles a “top hat” function, rather than a sinusoidal wave.
Also, in use of the apparatus described in WO 00/64038, energy is recovered from the sample once per cycle of the excitation field. In a first step of the cycle, the sample is magnetised, with very little corresponding temperature rise, as the sample is far from the ferromagnetic phase transition point of the sample. The sample is then demagenetised by the removal of the field from the sample, after which the temperature of the sample falls as thermal energy within the sample is expended in working to re-randomise the domains in the sample against the field arising from the remnant magnetism of the sample. After a short time, heat from the surroundings warms the sample, and the magnetic domains within the sample become randomly oriented, and this leads to the generation of an independent magnetic flux as the magnetic field arising from the alignment of the magnetic domains collapses. This independent magnetic flux delivers power to the field generation apparatus.